The present application is based on and claims priority under 35 U.S.C xc2xa7119 with respect to Japanese Patent Application No. 2001-369026 on Dec. 3, 2001 (13th Year of Heisei), the entire content of which is incorporated herein by reference.
1. Field of the Invention
The present invention is generally directed to a multi-stage vacuum pump and in particular to a multi-stage vacuum pump which is oil-free (dry) in their pumping chambers.
2. Prior Art
As Japanese Patent Publication No. 3051515 discloses, a conventional multi-stage vacuum pump of the type is constructed to have a plurality of in-series pumping chambers each of which accommodates a pair of intermeshing rotors which are all of a xe2x80x9cRootsxe2x80x9d-type profile and which are fixedly mounted on a pair of respective shafts. The pair of xe2x80x9cRootsxe2x80x9d-type profile rotors which are provided in each pumping chamber axe rotated therein to make a space evacuated which is connected to an inlet port or suck port of the pumping chamber by compressing a gas sucked from the space to be evacuated. While the rotors are being in rotation, a heat of compression is generated due to the gas compression. Such a compression heat is radiated from an outer surface of the housing to the atmosphere and is cooled down to a temperature by cooling water passing through a cooler secured, to the housing. Thus, the housing of the multi-stage vacuum pump becomes free from the possible temperature increase.
On the other hand, while the multi-stage vacuum pump is in operation, the pumping chambers are made substantially vacuumed, which causes less heat radiation to the gas in each of the pumping chambers from pair of xe2x80x9cRootsxe2x80x9d-type profile rotors and the respective pair of shafts, resulting in that each of the pair of xe2x80x9cRootsxe2x80x9d-type profile rotors and the respective pair of shafts becomes larger and larger in temperature increase degree when compared to the housing. Thus, the larger the temperature increase resulting from the compression heat, the larger the temperature difference between the housing and each of the pair of xe2x80x9cRootsxe2x80x9d-type profile rotors and the respective pair of shafts. Due to the fact that the housing is brought into thermal expansion in proportion to temperature increase, if the aforementioned temperature difference becomes larger above a specific value, the position of each of the pair of xe2x80x9cRootsxe2x80x9d-type profile rotors may vary relative to the housing.
In addition, though the multi-stage vacuum pump is designed to define, in each of the pumping chambers, an axial clearance having a fixed length between each of the xe2x80x9cRootsxe2x80x9d-type profile rotors and an inner surface of the pumping chamber, the fixed clearance length may become shorter and shorter due to the above-mentioned relatively large thermal expansion difference between the shaft and the housing. This results in, in extreme case, that the xe2x80x9cRootsxe2x80x9d-type profile rotors are brought into sliding engagement with the inner surface of the pumping chamber to generate uncomfortable noise or dreadful noise. Though enlarging or increasing the fixed clearance length makes the multi-stage vacuum pump free from such noises, the increased clearance length increases the amount of gas which flows back therethrough, resulting in lowering the total pump volume efficiency.
Thus, a need exists to provide a xe2x80x9cRootsxe2x80x9d-type multi-stage vacuum pump which is free from the above-described drawbacks.
Accordingly, in order to meet the above need to overcome the aforementioned drawbacks or problems, a first aspect of the present invention provides a multi-stage vacuum pump which comprises:
a housing in which a plurality of pumping chambers are formed, the pumping chambers being arranged in series and being in fluid communication with one another, one of the pumping chambers which is at one end of the series acting as an initial stage pumping chamber, another of the pumping chamber which is at the other end of the series acting as a final stage pumping chamber,
the housing being provided with an inlet port for sucking a gas from a space to be evacuated into the initial stage pumping chamber, the housing being provided with an outlet port for exhausting the gas from the final stage pimping chamber;
Roots-type pump sections occupying the respective pumping chambers, each of the Roots-type pump sections having a pair of intermeshed Roots-type profile rotors; and
a pair of shafts adapted for rotation within the housing about their lengthwise axes in contra-rotational direction, the pair of shafts being secured to the respective Roots-type profile rotors in each of the Roots-type pump sections, one end of each of the shafts being made immovable in its lengthwise direction, the other end of each of the shafts being made expandable in its lengthwise direction.
A second aspect of the present invention is to provide multi-stage vacuum pump whose gist is to modify the structure of the first aspect, wherein an axially defined clearance between an inner surface of each of the pumping chambers and the pair of the Roots-type profile rotors in such a manner that the closer to one end of the shaft, the smaller the clearance of the pumping chamber.
A third aspect of the present invention is to provide multi-stage vacuum pump whose gist is to modify the structure of the first aspect, wherein one end of the each of the shafts is positioned at a side of the final stage pumping chamber.